Render live transcript segments before persistence

[ComputelessComputer]

Show live-only transcript segments when no transcript row exists yet, and cover the partial-only renderer case.

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Note

Low Risk
Small UI rendering change in the desktop transcript viewer with no auth, persistence, or API changes in this diff.

Overview
Live STT text now appears in the transcript viewer before any persisted transcript ID is available, instead of rendering nothing when transcriptIds is empty.

TranscriptViewer derives visibleTranscriptIds from real IDs when present, otherwise uses a single placeholder id __live-transcript__ when liveSegments is non-empty. Rendering, separators, and “last transcript” live-segment attachment all use that list so auto-scroll and bottom-pinning behave the same as for a normal last row.

A unit test asserts that with empty transcriptIds and active live segments, RenderTranscript receives one live segment and the placeholder transcript id.

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